The "Let us vote" business, putting the best face on it, means that the citizens ought to be allowed to amend the state constitution. It is of course possible to amend the constitution and citizen votes decide whether an amendment will be adopted, but it is not as easy to amend the constitution in Iowa as it is in other states. I expect that the anti-gay forces will solicit the help of various powerful out-of-state groups to produce television commercials that would show the horrors associates with allowing two people of the same sex to have their marriage recognized by the state.
Putting a more realistic face on it, the "Let us vote" crowd espouses the view that a minority ought not to have rights unless those rights are conferred by the majority. And further, the majority can choose to refuse those rights based upon whim or overt bigotry. In other words, the "Let us vote" attitude is a "majority rules" mentality.